docs(package-json): document default main behavior

PR-URL: #2881
Credit: @klausbayrhammer
Close: #2881
Reviewed-by: @wraithgar

Author: klausbayrhammer

docs/content/configuring-npm/package-json.md

@@ -325,6 +325,8 @@ This should be a module relative to the root of your package folder.
 For most modules, it makes the most sense to have a main script and often
 not much else.
 
+If `main` is not set it defaults to `index.js` in the packages root folder.
+
 ### browser
 
 If your module is meant to be used client-side the browser field should be